As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, or a multi-function peripheral in which some functions of a printer, a facsimile machine, a copier, a plotter, and so forth, are combined, an ink jet recording apparatus or such, for example, is known as an image forming apparatus of a liquid discharge recording type using a recording head configured as a liquid discharge head (or liquid droplet discharge head) that discharges ink droplets. In the image forming apparatus of the liquid discharge recording type, ink droplets are discharged by a recording head to a sheet of paper that has been conveyed, and an image is formed on the sheet of paper. The sheet of paper that has been thus conveyed may include not only paper but also an OHP (Over Head Projector) sheet or such, and any thing capable of liquid being adhered thereto, and may also be referred to as a recording medium, recording paper, or such. Forming an image may also be referred to as recording, printing and so forth. The image forming apparatuses of liquid discharge recording type include a serial-type image forming apparatus and a line-type image forming apparatus. The serial-type image forming apparatus is such that a recording head moving in main scan direction discharges liquid droplets and forms an image. The line-type image forming apparatus is such that a line type recording head is used where the recording head not moving discharges liquid droplets and forms an image.
It is noted that in the present patent application, the “image forming apparatus” of liquid discharge recording type means an apparatus that discharges liquid to a medium such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, ceramics or such. “Forming an image” means not only giving to a medium an image that has a meaning such as a letter, a figure or such, but also giving to a medium an image that does not have a meaning such as a pattern or such (also merely causing a liquid droplet to land on a medium). “Ink” means not only one called “ink” but is used as a general term of any thing that is capable of being used to form an image and may be referred to as recording liquid, fixing solution, liquid or such. For example, a DNA sample, resist, pattern material, resin, and so forth, are included in “ink”. Further, an “image” is not only a planar image but also an image given to a thing that has been formed three-dimensionally, or a statue or such formed as a result of a shape being molded three-dimensionally.
As such an image forming apparatus, one is known in which a sub-tank (also referred to as a head tank, a buffer tank or such) is provided for supplying ink to a recording head, and the ink is supplied to the sub-tank from a main tank (also referred to as an ink cartridge) which is detachably loaded in a body of the image forming apparatus.
As such an image forming apparatus, one is known in which a sub-tank (also referred to as a head tank, a buffer tank or such) may have a negative pressure creating function (mechanism) that creates negative pressure for the purpose of preventing ink from seeping or dripping from nozzles of a recording head. The sub-tank has a flexible member (film member) used as one side of an ink container that contains ink, and a negative pressure creating part that includes a resilient member that gives such force to the flexible member to cause it to move outward. Further, an atmosphere opening mechanism is provided that is capable of being opened and closed, and opens the inside of the ink container to the atmosphere. In this configuration, ink is supplied to the recording head from the ink container.
The sub-tank is provided with a displacement member (also referred to as a detection member or a detection filler) which changes a position itself as a position of the flexible member changes. When an atmosphere opening filling process is to be carried out where the atmosphere opening mechanism of the sub-tank is opened and the ink is supplied from the main tank to the sub-tank, a carriage that carries the recording head and the sub-tank is moved to a predetermined detection position (i.e., a fully filling up detection position), and the sub-tank is opened to the atmosphere as a result of a driving part of the atmosphere opening mechanism being operated. In this state, supplying ink to the sub-tank is carried out in a state where the carriage has been moved to the predetermined carriage position. Then, when the displacement member is detected by a detection part of the image forming apparatus, it is determined that the sub-tank has been fully filled up (see the following Patent Documents 1-9).    Patent Document 1: Japanese Patent No. 4298474;    Patent Document 2: Japanese Patent No. 4190001;    Patent Document 3: Japanese Patent No. 4155879;    Patent Document 4: Japanese Laid-open Patent Application No. 2007-015153;    Patent Document 5: Japanese Laid-open Patent Application No. 2007-130979;    Patent Document 6: Japanese Laid-open Patent Application No. 2008-132638;    Patent Document 7: Japanese Laid-open Patent Application No. 2009-023329;    Patent Document 8: Japanese Laid-open Patent Application No. 2009-274325;    Patent Document 9: Japanese Laid-open Patent Application No. 2009-023092
In this case, in order to make it possible to supplementarily supply the ink even during printing operations, the following control may be carried out (see Patent Document 9). That is, when an ink consumption amount is equal to or more than a first predetermined value, the following operation is carried out. Based on information correlating to an ink supply amount having been supplied to the sub-tank from the main tank during printing, ink supply from the main tank to the sub-tank is carried out when the ink supply amount is equal to or less than a second predetermined amount. Ink supply from the main tank to the sub-tank is not carried out when the ink supply amount exceeds the second predetermined amount.
It is noted that ink supply to the sub-tank may be carried out even during printing operation by providing an ink remaining amount detection part to the sub-tank instead of the above-described configuration of the sub-tank (see the following Patent Document 10).    Patent Document 10: Japanese Patent No. 3219326
In the above-mentioned case where the displacement member that changes the position according to the ink remaining amount in the sub-tank is provided to the sub-tank while fully filling up of the sub-tank is detected by the body of the image forming apparatus, the carriage is to be moved to the predetermined fully filling up position when ink supply is to be carried out from the main tank to the sub-tank. Therefore, it is necessary to interrupt printing operations in order to carry out an ink supply operation when the ink remaining amount in the sub-tank is lowered during the printing operations. Thus, a printing speed may be lowered.
In this case, it may be possible to calculate an ink consumption amount in the sub-tank by counting the number of discharge droplets or so, and ink supply from the main tank to the sub-tank may be carried out by a supply amount corresponding to the calculated ink consumption amount. However, in this method, since detection of ink fully filling up of the sub-tank is carried out not so precisely, excessive negative pressure in the sub-tank due to shortage of ink supply or insufficient negative pressure due to excess of ink supply may occur. In order to avoid such a situation, it is necessary to periodically carry out the atmosphere opening filling process after moving the carriage to the fully filling up detection position. Thus, a printing operation is to be interrupted, and a printing speed may be lowered.
Further, it may be possible to provide to the carriage a part to detect an ink remaining amount in the sub-tank and, a part to drive the atmosphere opening mechanism, and provide to the carriage necessary members and parts to control ink supply to the sub-tank. However, in this method, the carriage may becomes heavy, a size of the carriage may be increased, and thus, a size of the image forming apparatus may be increased.